Oxygenic photosynthesis depends on the absorption of sunlight by auxiliary light-harvesting pigments, which are incorporated within the holocomplexes of photosystem-I and photosystem-II. In each photosystem (PS), sizable arrays of chlorophylls and other accessory pigments (e.g., carotenoids) act cooperatively as antennae for the collection of light energy and as a conducting medium for excitation migration toward a photochemical reaction center (see, e.g., Emerson & Arnold, J Gen Physiol 15: 391-420, 1932; Emerson & Arnold, J Gen Physiol 16: 191-205, 1933; Gaffron & Wohl, Naturwissenschaften 24: 81-90, 1936; Melis, In, Oxygenic Photosynthesis: The Light Reactions” (DR Ort, CF Yocum, eds), Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 523-538, 1996). Organized as distinct pigment-protein complexes and contained within PSI and PSII, these light-harvesting antennae perform the functions of light absorption and excitation energy transfer to a photochemical reaction center (see, e.g., Simpson and Knoetzel, In: Ort DR and Yocum CF (eds), Oxygenic Photosynthesis: The Light Reactions, pp. 493-506, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1996; Pichersky and Jansson, In: Ort DR and Yocum CF (eds), Oxygenic Photosynthesis: The Light Reactions, pp. 507-521, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1996). Up to 350 chlorophyll a (Chl a) and Chl b molecules can be found in association with PSII, whereas the Chl antenna size of PSI may contain up to 300 mainly Chl a molecules (Melis, Biochim. Biophys. Acta (Reviews on Bioenergetics) 1058: 87-106, 1991; Melis, In, Oxygenic Photosynthesis: The Light Reactions” (DR Ort, CF Yocum, eds), Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 523-538, 1996). Some of these Chl molecules are contained within the PS-core complexes, which are highly conserved in all organisms of oxygenic photosynthesis. The PSII-core complex contains about 37 Chl a molecules, whereas the PSI-core complex contains 95 Chl a molecules (Glick & Melis, Biochim Biophys Acta 934: 151-155, 1988; Jordan et al., Nature 411(6840): 909-917, 2001; Zouni et al., Nature 409: 739-743, 2001; Ruban et al., Nature 421: 648-652, 2003). In green plants and algae, the remaining Chl a and Chl b antenna molecules are organized within 10 peripheral subunits of the so-called auxiliary chlorophyll a-b light-harvesting complex. There are six such subunits for PSII (Lhc b1-b6) and four for PSI (Lhc a1-a4) (Jansson et al., Plant Mol Biol Rep, 10: 242-253, 1992). These peripheral Lhc subunits are not essential for the process of photosynthesis. Indeed, when the chloroplast development is limited, stable assembly of the PSII-core and PSI-core complexes takes place in the absence of any Lhc proteins (Glick & Melis, 1988, supra).
A genetic tendency of photosynthetic organisms to assemble large arrays of light absorbing Chl antenna molecules in their photosystems is a survival strategy and a competitive advantage in the wild, where light is often limiting (Kirk, Light and photosynthesis in aquatic ecosystems, 2nd edn. Cambridge University Press, Cambridge, England, 1994). However, the Chl antenna size of the photosystems is not fixed but can vary substantially depending on developmental, genetic, physiological and even environmental conditions (Melis, 1991, supra). It is recognized in the field that a genetic regulatory mechanism dynamically modulates the Chl antenna size of photosynthesis (Anderson, Annu Rev Plant Physiol 37: 93-136, 1986; Escoubas et al., Proc. Nat. Acad. Sci. 92: 10237-10241, 1995; Melis, 1991 and 1996, both supra; Melis, Intl. J. Hydrogen Energy 27: 1217-1228, 2002; Melis, Chapter 12 in Artificial Photosynthesis: From Basic Biology to Industrial Application, A F Collins and C Critchley (eds.), Wiley-Verlag & Co., pp. 229-240, 2005). For example, the Chl antenna size is adjusted and optimized in response to the light intensity during plant growth (Ley and Mauzerall, Biochim Biophys Acta 680: 95-106, 1982; Sukenik et al., Biochim Biophys Acta 932: 206-215, 1988; Smith et al., Plant Physiol. 93: 1433-1440, 1990; LaRoche et al., Plant Physiol 97: 147-153, 1991; Maxwell et al., Plant Physiol 107: 687-694, 1995; Falbel et al., Plant Physiol. 112: 821-832, 1996; Webb and Melis, Plant Physiol. 107: 885-893, 1995; Ohtsuka et al., Plant Physiol. 113: 137-147, 1997; Tanaka and Melis, Plant Cell Physiol. 38: 17-24, 1997; Masuda et al., Plant Physiol. 128: 603-614, 2002). Physiological and biochemical consequences of the function of this molecular regulatory mechanism for the Chl antenna size are well understood. However, little is known about the genes and proteins and their mode of action in this regulation. The Chl antenna size regulatory mechanism is highly conserved and functions in all organisms of oxygenic and anoxygenic photosynthesis (Anderson, Annu Rev Plant Physiol 37: 93-136, 1986; Nakada et al., J Ferment Bioengin 80: 53-57, 1995; Escoubas et al., Proc. Nat. Acad. Sci. 92: 10237-10241, 1995; Huner et al., Trends in Plant Science, 3: 224-230, 1998; Yakovlev et al., FEBS Lett 512: 129-132, 2002; Masuda et al., Plant Physiol. 128: 603-614, 2002; Masuda et al., Plant Mol. Biol. 51: 757-771, 2003). Thus, identification of the relevant genes and elucidation of the genetic mechanism for the regulation of the Chl antenna size in Chlamydomonas reinhardtii can apply to all photosynthetic organisms.
Although a smaller Chl antenna size may compromise the ability of a plant, e.g., algae, to survive in the wild, in a high-density cultivation environment, a smaller chlorophyll antenna size would help to diminsh the over-absorption and wasteful dissipation of excitation energy by the first layer of leaves, cells or chloroplasts, and would also help diminish photoinhibiton of photosynthesis at the surface while permitting greater transmittance of light deeper into the culture. Such altered optical properties of the cells would result in greater photosynthetic productivity and enhanced solar conversion efficiency by the high-density culture.
Previous work (Masuda et al. 2003, supra; Polle et al., Planta 217: 49-59, 2003, Melis, 2005, supra) described the isolation of tla1, a Chlamydomonas reinhardtii DNA insertional mutant having a truncated light-harvesting chlorophyll antenna size (Polle et al, 2003, supra). Although these studies identified a mutant that had reduced antenna size, there was no teaching of whether the phenotype was associated with increased or suppressed tla1 expression. Accordingly, there is a need for further elucidation of mechanism of Tla1-mediated changes in chlorophyll antenna size.